The present invention relates generally to communications systems and, more particularly, to discrete multi-tone communications systems and digital subscriber line communications systems.
Remote client personal computers (PCs) are typically connected to the Internet over a telephone network of twisted-pair transmission lines via analog modems, as illustrated in FIG. 1. Unfortunately, analog modem speeds are currently limited to 56 thousand bits per second (56 Kbps) because of various constraints of existing telephone networks.
Asymmetrical Digital Subscriber Line (ADSL) technology is currently being developed to extend bandwidth over twisted-pair transmission lines by an order of magnitude. ADSL is based on a discrete multitone (DMT) transmission system. ADSL transmission rates over twisted pair transmission lines can reach as high as 8 million bits per second (8 Mbps) downstream (i.e., towards a client PC) and up to 256 Kbps upstream (towards a server). Data transmission rates for ADSL are substantially higher in the downstream direction than in the upstream direction. The asymmetric nature of ADSL fits well with data delivery requirements of the Internet. More data is typically transmitted downstream to a client PC than upstream from a client PC to a server.
Unfortunately, a significant problem currently plaguing the implementation of ADSL is the bandwidth burden placed on a telephone network by ADSL as illustrated in FIG. 2. In FIG. 2, lines 20 correspond to individual analog lines, such as local subscriber loops or lines and plain old telephone system (POTS) connections to a telephone company central office (CO) switch. Lines 22 correspond to T-1 lines (i.e., lines having bandwidth capabilities of 1.544 Mbps and capable of carrying 24 digitized voiceband connections). Lines 24 correspond to T-3 lines (i.e., lines having bandwidth capabilities of 28 T-1 lines and that are time-division multiplexed).
Although 24 analog POTS connections can be handled by a single T-1 line, 24 ADSL connections utilize almost the entire bandwidth of a T-3 line. This bandwidth constraint is further complicated by the fact that many analog subscriber lines may not terminate at a CO switch. Analog subscriber lines may terminate in remote locations, such as a manhole or utility box in a neighborhood, where a subscriber line is digitized and then routed to a CO switch via a T-1 or T-3 line. Increasing bandwidth for these remote links may be costly and may complicate implementation of ADSL.
In addition, current ADSL implementations typically include a dedicated pair of ADSL modems for each subscriber line. This is in contrast to voice-band modems, where a server only utilizes dedicated modems for each active connection. Requiring a dedicated pair of ADSL modems for each subscriber line may add significantly to the cost of ADSL implementation by telephone service providers.
Various solutions have been proposed to alleviate bandwidth constraints and thereby facilitate implementation of ADSL technology. One solution involves statistical multiplexing at the remote site. Another solution proposes terminating multiple ADSL subscriber lines in a single shared modem using common parameters for each connection. This may simplify hardware requirements on the telephone network side of each ADSL subscriber line because each subscriber line may share a high-speed transmitter. A single shared modem eliminates the need for separate ADSL modems for respective ADSL subscriber lines.
Unfortunately, a limitation of a single shared ADSL modem is that modems on the other end of subscriber lines connected to the shared modem may need to receive the same signal. Accordingly, the bandwidth of the transmitted signal within each frequency band may need to be reduced to accommodate the subscriber line in the group with the lowest bandwidth within each of the frequency bands. Accordingly, the net bandwidth that can be exploited by a shared modem on the server end may be less than that of a single modem on the worst line.
Various ways of overcoming limitations with shared ADSL modems have been proposed. For example, U.S. Pat. No. 5,557,612 to Bingham relates to a system for establishing communications between a central unit and a plurality of remote units wherein access requests include an identification of each remote unit. A particular subchannel is allocated to the remote units by the central unit in an ADSL application. Units requiring bandwidth already allocated to other clients may be denied service. U.S. Pat. No. 5,608,725 to Grube et al. relates to establishing a communications system wherein a primary site is coupled to a plurality of secondary sites via low pass transmission paths. However, only a single secondary site may be active at a time. Furthermore, the primary site allocates carrier channels of the inbound and outbound low pass transmission paths.
In view of the above discussion, it is an object of the present invention to facilitate the implementation of ADSL by including statistical multiplexing within an ADSL modem.
It is another object of the present invention to facilitate the implementation of ADSL without requiring a separate ADSL modem at each end of a subscriber loop.
It is another object of the present invention to help lower costs associated with ADSL implementation.
These and other objects of the present invention are provided by systems, methods and computer program products for simultaneously transmitting data over a plurality of subscriber lines (e.g., twisted pair telephone wires) extending between a shared device and a respective plurality of remote devices using a symbol-based discrete multi-tone transmission scheme, wherein a unique destination code is assigned to each of the remote devices. A destination code may be assigned to each remote device statically or dynamically during a communication handshake between each remote device and the shared device. An exemplary shared device with which the present invention may be utilized is a shared access ADSL modem. Exemplary remote devices with which the present invention may be utilized are ADSL modems.
Operations may include training each connection over each subscriber line to provide subscriber line specific information for each subscriber line, and transmitting and receiving information over each subscriber line utilizing the corresponding subscriber line specific information. During each symbol interval, a destination code within a first group of frequency bands and modulated data within a second group of frequency bands different from the first group of frequency bands are transmitted from the shared device, via the plurality of subscriber lines, to the remote devices.
The number of bits per frequency band for the first group of frequency bands is selected as being receivable by each of the remote devices. The number of bits per frequency band for the second group of frequency bands is selected to maximize data transmission bandwidth for a subscriber line connected to a remote device having the destination code within the first group of frequency bands. The number of bits per frequency band for the second group of frequency bands for a remote device may be selected during a communication handshake between a remote device and a shared device by probing a subscriber line connected to the remote device to determine data rates that can be supported by the subscriber line at each of a plurality of frequency bands.
Operations for transmitting and receiving information over each subscriber line may include equalizing a received signal from a subscriber line utilizing the subscriber line specific information to provide client specific equalization coefficients. Operations for transmitting and receiving information over each subscriber line may include detecting symbols in the frequency domain representation of a signal received from a subscriber line utilizing the subscriber line specific information to provide client specific frequency information which defines the range of symbols encoded for each frequency.
In addition, operations for transmitting and receiving information over each subscriber line may include encoding, decoding, interleaving, deinterleaving, scrambling and descrambling the symbols of a signal received from a subscriber line utilizing the subscriber line specific information. Also, operations for transmitting and receiving information over each subscriber line may include mapping symbols to be transmitted on a subscriber line utilizing the subscriber line specific information.
According to another aspect of the present invention, systems, methods and computer program products are provided for receiving, at a remote device, such as an ADSL modem, data transmitted over a subscriber line extending from a shared device using a symbol-based discrete multi-tone transmission scheme, wherein the remote device has a unique destination code assigned thereto. A destination code may be assigned to each remote device statically or dynamically during a communication handshake between each remote device and a shared device. At each remote device receiving the transmitted first and second groups of frequency bands, the transmitted first group of frequency bands is examined for a destination code therewithin. At a remote device having an assigned destination code that matches a destination code within the transmitted first group of frequency bands, modulated data within the transmitted second group of frequency bands is demodulated. The number of bits per frequency band for the second group of frequency bands for the remote device may be selected during a communication handshake between the remote device and a shared device by probing the subscriber line (e.g., a twisted pair telephone wire) connected to the remote device to determine data rates that can be supported by the subscriber line at each of a plurality of frequency bands.
According to another aspect of the present invention, systems, methods and computer program products are provided for simultaneously transmitting data over a plurality of subscriber lines, such as twisted pair telephone wires, extending between a shared device and a respective plurality of remote devices using a symbol-based discrete multi-tone transmission scheme. According to the present invention, a destination code within a first group of frequency bands and modulated data within a second group of frequency bands different from the first group of frequency bands, are transmitted from the shared device to remote devices. The number of bits per frequency band for the second group of frequency bands is selected for a subscriber line connected to a remote device having the destination code within the first group of frequency bands. The number of bits per frequency band for the second group of frequency bands for a remote device is selected during a communication handshake between the remote device and the shared device by probing a subscriber line connected to the remote device to determine data rates that can be supported by the subscriber line at each of a plurality of frequency bands.
The use of different downstream frequency band allocations for each remote device permits optimum performance for each subscriber line even though a shared ADSL modem is utilized. Accordingly, the present invention can overcome significant limitations in the throughput capabilities of shared ADSL modems. As a result, costs associated with ADSL implementation can be lowered because a separate ADSL modem is not required at each end of a subscriber line.